Looks like Rockstar have released a small game update for the PC version of GTA IV. Next time you log on to Games for Windows LIVE you'll be prompted to download it. The update includes everything from the previous patches, so if you don't already have the 188.8.131.52 patch you'll only need to install this one, not all of them.
Rockstar haven't yet added this to the official site yet, so we're not sure of the exact details of what it does. However, some players are reporting performance increases with an extra 5 fps, sometimes even more. Guess it all depends on your setup. Let us know in the comments how the patch effects your game. We'll post full details of the changelog once Rockstar put it up.
UPDATE: It's simply a maintenance update:
* -Resource management adjustments based on the operating system detected.
As an aside: Both ATi and nVidia this week released updates to their graphics card drivers. These can of course also help give you better performing games, so it's recommended you upgrade these as well.
EDIT: Local mirror of the patch here: download patch 184.108.40.206
Here's a quote from Rockstar Toronto at GTAF:
Today we released a Maintenance Update for the PC version of Grand Theft Auto IV. This update ensures all copies of the game are properly aligned for the next gold master build pressing. We have also improved the resource management numbers to detect and adjust accordingly to different operating systems.
Like all previous updates, this contains all the fixes from past title updates, so there is no need to re-install previous patches if you have to re-install the game for any reason. A direct link to download Maintenance Update 220.127.116.11 can be found here. For more details and download links for other languages, check out the Grand Theft Auto IV PC support page.
This means they may add DX10 support or some big, big improvements.